Astronomy Now Online

Top Stories

Lazy pulsar powered by companion

...XMM-Newton has, for the first time, detected X-ray pulses emanating from both stars of a closely packed binary pulsar system, revealing two extremely dense rapidly rotating neutron stars....

read more

Black holes have simple feeding habits ...the results of a massive observing campaign of spiral galaxy M81 have shed light on the feeding habits of different sized black holes, and provide a benchmark for predicting the properties of a new class of black hole...

read more

Earth's laws still apply in distant Universe of the most important numbers in physics, the proton-electron mass ratio, is the same in a galaxy six billion light years away as it is here on Earth, laying to rest debate about whether the laws of nature vary in different places in the Universe...

read more

Spaceflight Now +

Subscribe to Spaceflight Now Plus for access to our extensive video collections!
How do I sign up?
Video archive

STS-120 day 2 highlights

Flight Day 2 of Discovery's mission focused on heat shield inspections. This movie shows the day's highlights.


STS-120 day 1 highlights

The highlights from shuttle Discovery's launch day are packaged into this movie.


STS-118: Highlights

The STS-118 crew, including Barbara Morgan, narrates its mission highlights film and answers questions in this post-flight presentation.

 Full presentation
 Mission film

STS-120: Rollout to pad

Space shuttle Discovery rolls out of the Vehicle Assembly Building and travels to launch pad 39A for its STS-120 mission.


Dawn leaves Earth

NASA's Dawn space probe launches aboard a Delta 2-Heavy rocket from Cape Canaveral to explore two worlds in the asteroid belt.

 Full coverage

Dawn: Launch preview

These briefings preview the launch and science objectives of NASA's Dawn asteroid orbiter.

 Launch | Science

Become a subscriber
More video

Twin galaxies in cosmic

arm wrestle

Posted: June 27, 2008

Astronomers at the Gemini Observatory have captured the early stages of gravitational interaction between two similar galaxies as they tug on each other’s spiral arms.

The galaxies – NGC 5427 and NGC 5426, which are collectively known as Arp 271 – are around 90 million light years away in the constellation of Virgo. Their once near-identical masses, structures and shapes have already begun to distort as the mutual wrench of gravity overpowers their outer shells in a galactic embrace that could take a hundred million years to complete.

Appearances can be deceiving; in the image above, NGC 5427, the ‘face-on’ galaxy, is farthest away, while the oblique galaxy NGC 5426 is nearest. Over time, NGC 5427 will revolve around its twin, bringing it into the foreground. Image: Gemini Observatory.

A funnel of gas bridges the two galaxies, allowing the twins to feast on shared gas and dust across the 60,000 light years that currently separates them. The colliding gas also appears to have sparked localised bursts of star formation, including dense and curiously knotted pockets of stellar nurseries in NGC 5426, and a frenetic rate of star formation in NGC 5427, much more than expected for a typical galaxy of this type.

Even more astounding is the unusually straight western (top) arm of NGC 5427. It is as if the strong tidal forces have brutally snapped the arm in two, causing it to bleed starlight into the black canvas of space. Like watching a slow motion replay of two skaters spinning on ice, NGC 5426 has grabbed its twin by the arm and has begun to spin it around in a manoeuvre that will take millions of years to execute.

Once thought to be a rare event, gravitational interactions between galaxies are now known to be rather common, as highlighted by the breath-taking gallery of interacting galaxy images released by Hubble in April to mark its 18th anniversary. Such cosmic tug-o-wars are especially common in densely populated galaxy clusters and are considered to play an important role in galaxy evolution. Indeed, our own Galaxy will ultimately meet its fate with the Andromeda Galaxy, and the events played out within the Arp 271 system could be a sneak preview of what is to come.